For sure they making money and made a lot in the last years with selling players , but mostly they have to sell them because of there higher salary they want.
And on top of that they want trophies but they got unlucky and other teams was better in that seasons.
If they sign the players at a tender age and when they are not in hot form there is no way the players will be demanding high salaries when their contract is still running in the team, I understand that Dortmund wants to keep making money since they can not win trophies with the players they have but I think they can still have good players after selling some all they should do is to spend money to buy young players then groom some to sell and some to fight for the trophy.
Dortmund at a time made it known that it is a business for them to sell players as they make food money from it, I think the money they make from it sometimes is enough to not bother them about trophies, players salaries may not be a serious issue for Dortmund, they are not poor neither have they declared bankruptcy before so it is clear it was just business for them but they still keep the hope of winning the trophy alive actually. The fact that a player asked for higher py has not been a reason Dortmund will let such player go and again I agree that these players they get at tender age who has no solid for at the time, will only sit for their contract and no lt demand higher salaries.
No they never made clear that buying and selling players is their bread and butter business, they never did. But it is the way things normally go for the clubs that are not super rich, but rich enough to sign those upcoming stars. They know every time that the players will soon be gone and in the case of Dortmund, they are a publicly listed company, which entails obligations like corporate risk management and focus on financial solidity and earnings. While other clubs can refuse high offers, it is not that easy for Dortmund as they have to justify it in front of their shareholders.
Dortmund is not the only club losing the best players again and again. It is quite normal that when Bayern comes by and wants to pay a player 15-20 million a year, Dortmund can't do it, or only for a very small number of players. If they could, they would have not sold so many of their players.